This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Appln. No. 01810642.7 filed in Europe on Jul. 2, 2000; the entire content of which is hereby incorporated by reference.
1. Technical Field
The invention is directed to a fuse having two spaced-apart power supply connections and an active part with a fusible, current-carrying fuse element and with an arc-extinguishing medium. The fuse element is connected in an electrically conducting manner to the two power supply connections and is arranged on an electrically insulating substrate. It is of a modular construction and has modules connected in series. The exposed surfaces of the fuse element are covered by arc-extinguishing medium. A fuse of this type is preferably used in medium- and high-voltage systems and in these systems, but also in low-voltage systems, can interrupt strong short-circuit currents and sustained weak overload currents.
2. Prior Art
A fuse of the type stated at the beginning is described, for example, in DE 198 24 851 A1 and U.S. Pat. No. 4,638,283, A. This fuse serves in particular for protecting circuits with high currents and voltages. In the case of this fuse,-a fuse element takes the form of a wire strip. The wire strip is wound onto an asymmetrically configured electrically insulating supporting body. Formed into the wire are evenly spaced-apart constrictions. The two ends of the strip are respectively connected to one of two power supply connections of the fuse. The wound supporting body is accommodated in a housing filled with arc-extinguishing medium, such as quartz sand in particular. When an inadmissible current occurs, the wire is interrupted at the constrictions by melting or by firing of explosive charges. Arcs caused as a result are suppressed by the extinguishing medium.
This fuse is of a relatively complex construction and can only be produced by a large number of working steps. The fuse is therefore relatively expensive. During operation of the fuse, the complex construction of the fuse reduces quite significantly the outward flow of the heat generated by the operating current in the fuse wire. The operating temperature of the fuse is considerably increased in this way. Therefore, the melting temperature of the fuse wire is often already reached when the operating current is still below the permissible current. The arcs occurring when there is this small current contain only a relatively small amount of energy, which may not be sufficient to melt the surrounding sand and interrupt the current by extinguishing the arcs.
Fuses for low voltages, generally of less than 110 V, and for small currents, generally of less than 5 A, are described in U.S. Pat. No. 5,479,147 A, GB 2,110,485 A, WO 89,08925 A, U.S. Pat. Nos. 6,034,589, 5,453,726 and DE 44,16,093 A. These fuses are constructed in the manner of a sandwich and comprise an insulating supporting body, applied to which is an intermediate layer which has a module or a plurality of parallel-connected modules of a fuse element. The sandwich is closed off by a layer which consists of arc-extinguishing material and is arranged on the intermediate layer. These fuses are intended in particular for use in electronic circuits and, without measures to eliminate the risk of an explosion, can only be used in the range of low power outputs.
The invention, as it is specified in the patent claims, is based on the object of providing a fuse of the type stated at the beginning which is distinguished in the high power-output range by simple construction and favorable triggering characteristics.
In the case of the fuse according to the invention, the active part is formed in the manner of a sandwich and has two stable moldings and a predominantly planar intermediate layer arranged between the moldings. The intermediate layer contains at least two series-connected first modules, increasing the holding voltage, whereas a first molding of the two moldings is formed at least by part of the electrically insulating substrate and the second molding is formed at least by part of the arc-extinguishing medium. On the basis of this design, the active part can now be provided directly with the power supply connections. It is therefore possible to dispense with an otherwise customary fuse housing, since compressed gas produced during triggering of the fuse according to the invention under the effect of the arc is taken up by the surrounding stable moldings.
The planar intermediate layer acting as the fuse element is arranged between stable moldings. Therefore, the fuse element, relieved of mechanical supporting functions, can be designed in virtually any desired way without having to take mechanical requirements into account. Consequently, current path structures made up in any desired way, giving the fuse particularly good triggering properties, can be provided in the fuse element. Since the fuse element is merely surrounded by stable moldings, the heat occurring in the fuse element during the operation of the fuse can be dissipated outward directly by the active part. Moderately heat-conducting quartz sand, otherwise customarily provided as the arc-extinguishing medium, and a housing that additionally hinders heat dissipation are no longer required in the case of the fuse according to the invention. The good heat dissipation from the active part and the simple and precise way in which the fuse element formed as an intermediate layer can be made up improve quite significantly the triggering characteristics of the fuse according to the invention in comparison with a fuse of a customary type of construction.
To increase the power taken up by the fuse, at least two second modules of the fuse element, which are connected in parallel with the first modules, should be additionally provided in the intermediate layer.
The modules are generally of an identical construction and in each case comprise a fuse wire configured in the form of a strip, with a constriction formed between the two ends of the strip. This ensures particularly simple production and activation of the fuse.
The fuse is distinguished by high mechanical strength if the fuse wire is applied to a planar surface of a first molding made of ceramic or glass. Preferably, the ceramic predominantly comprises aluminum oxide, beryllium oxide or aluminum nitride and the glass predominantly comprises borosilicate glass, since these materials conduct heat relatively well and consequently dissipate outward the heat formed in the fuse wire. At the same time, the fuse wire can be applied to the planar surface in a particularly simple way.
The fuse wire is advantageously formed by printing a curable metal paste onto the planar surface of the first molding and by curing the printed-on metal paste. Such technology is particularly advantageous for mass production and reduces the manufacturing costs of the fuse quite significantly. Moreover, this technology makes it possible for fine wire structures to be manufactured with exactly defined cross-sectional dimensions at the constriction of the fuse wire. Such wire structures are essential for good triggering characteristics of the fuse according to the invention. In addition, the fuse wire formed by printing on and curing the metal paste should be heat-treated at a temperature lying slightly below the melting temperature of the metal, since the fuse wire then has great dimensional stability and very constant electrical conductivity, which properties contribute significantly to favorable triggering characteristics.
A fuse with small dimensions and with an advantageously rapid outward flow of heat from the fuse element is achieved if the first molding is formed as a plate. If the fuse wire is applied to the upper side and underside of the plate, the dimensions of the fuse can be additionally reduced. The plate bearing the fuse wire is then covered on the upper side and on the underside by one of two second moldings.
A particularly compact fuse is distinguished by additionally being provided with at least one further first molding, which bears the fuse wire on a planar surface and is arranged with the planar surface on one of two second moldings.
To be able to disconnect even very high short-circuit currents with great certainty, it is recommendable to form in a surface of the second molding adjacent to the intermediate layer at least one material recess for receiving an arc-extinguishing medium in powder form.
The second molding should comprise a crosslinked silicone polymer or a mixture of crosslinked silicone polymers in which a filler based on a mineral compound or a mixture of a plurality of mineral compounds in powder form is embedded. The fuse according to the invention then has particularly great reliability. This is brought about in particular by the material of the arc-extinguishing medium melting at relatively low temperatures in comparison with the quartz sand otherwise customarily used, and then absorbing energy from the switching arcs formed during triggering of the fuse according to the invention, whereby the arc is rapidly and reliably extinguished.
For the fuse according to the invention to have a sufficiently good effect, the proportion of the filler in the silicone polymer should lie in the range from 5% by weight to 95% by weight, preferably in the range from 40% by weight to 85% by weight, and in particular in the range from 60% by weight to 80% by weight, calculated on the basis of the total weight of filler and polymer, and the filler should have an average particle size in the range from 0.5 to 500 xcexcm, preferably in the range from 10 to 250 xcexcm and specifically in the range from 20 to 150 xcexcm, preferably in the range from 30 to 130 xcexcm, or in the range from 0.5 to 50 xcexcm, preferably in the range from 0.5 xcexcm to 10 xcexcm.
Particularly suitable fillers are metal oxides, preferably aluminum oxide and/or titanium oxide, glasses, mica, ceramic particles, boric acid, metal hydroxides, preferably aluminum hydroxide and/or magnesium hydroxide, and/or mineral substances containing hydrate water, preferably based on aluminum oxide and/or magnesium oxide and/or magnesium carbonate.